1. Field of the Invention
The invention relates to the manufacture of ceramic tubes and, more particularly, to a method and apparatus for manufacturing ceramic tubes on a substantially continuous basis.
2. Description of the Prior Art
Ceramic tubes are used in heat exchangers where corrosive liquids or gases are handled, in high-temperature applications such as recuperators, in certain types of electrolytic cells, and in various other applications. Ceramic tubes currently are manufactured from ceramic materials such as sintered alpha silicon carbide, sintered aluminum oxide, sintered zirconia, and various others. Ceramic tubes are manufactured in a variety of diameters and wall thicknesses, and some currently are manufactured with longitudinal internal fins for enhanced surface area.
Ceramic tubes presently are manufactured by a so-called batch process wherein a series of separate steps are performed upon individual tubes. Unfortunately, batch-produced tubes cannot be manufactured in lengths any longer than approximately 14 feet due to various equipment limitations and to processing limitations including the cumulative length shrinkage. If long tubes (over about 14 feet) are being manufactured, the equipment needed to manufacture the tubes becomes very expensive. Also, it is possible to have differential properties from one end of the tube to the other as the length of the tube is increased. An additional drawback of the batch process is that damage can occur to tubes in process because the tubes must be handled frequently, that is, they must be moved from station-to-station during the manufacturing process. Additional drawbacks associated with batch-manufactured ceramic tubes include a long manufacturing time, the inability to rapidly feed back quality control information from finished tubes to tubes being processed, and a lack of optimum product quality.
Patents disclosing various batch processes for the manufacture of ceramic tubes include the patent to Jones, U.S. Pat. No. 3,950,463, and the patent to Dias, et al., U.S. Pat. No. 4,265,843. Jones discloses the production of beta alumina ceramic tubes wherein tubes of a fixed length, for example 18 inches, are passed at a uniform rate through an electric inductive furnace of open-ended tubular form. The temperature of the tube is raised within a short zone into the range of 1600.degree.-1900.degree. C. so that the tube is rapidly sintered, and thereafter is rapidly cooled. The patent to Dias, et al. similarly operates on tubes of fixed length, for example 20 centimeters. Dias, et al. disclose contacting a fixed length carbon-containing preform with elemental silicon powder at high temperature to transform at least a major part of the carbon to silicon carbide. This is known as reaction bonding, and is considered different from sintering by those skilled in the field of ceramics. Not only do the Jones and Dias et al. manufacturing processes suffer from the drawbacks of batch manufacturing processes, but they also are limited to relatively short lengths of tubes.
Other batch processes are known that are suitable for the manufacture of ceramic tubes, and the use of a variety of materials in such processes also is known. For example, U.S. Pat. Nos. 4,124,667; 4,179,299; 4,312,954; and 4,346,049, all issued to Coppola, et al., the disclosures of which are incorporated herein by reference, disclose sintered alpha silicon carbide ceramic bodies that can be injection molded on a batch basis. The ceramic bodies are manufactured from a mixture including silicon carbide, a carbon source, a boron source, a temporary binder, and a solvent.
The patent to Storm, U.S. Pat. No. 4,207,226 discloses a ceramic composition suited for injection molding and sintering, which composition includes, among other constituents, minor amounts of organo-titanates which materially reduce the viscosity of the composition. The patents to Ohnsorg, U.S. Pat. Nos. 4,144,207 and 4,233,256, disclose a composition and process for injection molding ceramic materials wherein a particular ceramic mixture includes, among other constituents, a combination of thermoplastic resin and oils or waxes. Although the Storm and Ohnsorg patents disclose ceramic compositions having desirable properties, they fail to teach or suggest any technique for overcoming the drawbacks of batch manufacturing processes.
Desirably, it would be possible to manufacture ceramic tubes more or less continuously so that tubes of essentially endless length could be manufactured and then cut to whatever length (for example, up to 60 feet or more) may be desired. It also would be advantageous to manufacture ceramic tubes by reducing handling damage, by providing a high degree of symmetry to the processing of the tubes at each stage, and by permitting rapid feedback of final product quality data to the early stages of the manufacturing process.